Acetylcholine (ACh) receptors in rat diaphragm muscle were blocked by intrathoracic injection of alpha-bungarotoxin (alpha-BuTx) or [125I]alpha-bungarotoxin ([125I]alpha-BuTx). The stability in vivo of the toxin-receptor complex formed by receptors in normal muscles and receptors in extrajunctional regions of denervated muscles was compared. Toxin was lost from junctional regions of normal muscles with a half-time of approximately 6 days. The loss of toxin was accompanied by a corresponding increase in the number of free toxin-binding sites. In contrast, 65% of the toxin bound to extrajunctional regions of denervated muscle was lost in 24 hr. 2. In a second series of experiments, animals were injected with [125I]alpha-BuTx and the muscle subsequently cultured for 24 hr. Loss of toxin again occurred more rapidly from extrajunctional receptors than from junctional receptors. The loss from extrajunctional receptors was described by a single first-order rate constant whose corresponding half-time was 8-11 hr. Loss was almost completely blocked by sodium cyanide and dinitrophenol and was inhibited by puromycin and cycloheximide. The radioactivity recovered in the medium was largely monoiodotyrosine. These results are consistent with the hypothesis that toxin loss reflects intracellular degradation of toxin-receptor complex. 3. Neonatal rats were injected with [125I]alpha-BuTx and the diaphragms cultured. Radioactive toxin was lost rapidly from extrajunctional regions of muscle and more slowly from regions containing end-plates. 4. These results could be explained by a difference in turnover rates for junctional and extrajunctional receptors.